The present invention relates to a corrosion inhibiting conductive coating which provides EMI/RFI shielding and prevents corrosion when applied to conductive substrates.
When dissimilar metal and/or conductive composite substrates are joined together in the presence of an electrolyte, a galvanic potential is established at the interface between the substrates. If this assembly is exposed to the environment, particularly under severe conditions such as salt fog or salt fog containing a high concentration of SO.sub.2, corrosion of the least noble of the substrates will occur, causing failure of the assembly. Mechanisms other than the establishment of galvanic potentials, e.g. crevice corrosion, may also cause corrosion and eventual failure of the part. In these cases one or all of the exposed materials in the assembly may corrode.
Thus, it has long been an object, especially among manufacturers of aircraft mainframes, ships, telecommunication equipment and other such equipment which is to be exposed to the environment, to prevent corrosion at there interfaces.
Further, it is often necessary to provide EMI/RFI shielding to the equipment described above, as aircraft, ships, etc, frequently utilize electronic systems which either radiate or are susceptible to electromagnetic or radio frequency interference (EMI/RFI).
Various methods have been attempted to achieve one or both of these objectives. Conventional shielding gaskets have been used in these applications to provide EMI/RFI shielding. These gaskets generally provide satisfactory shielding, but do not address the problem of corrosion. In fact, when the metal filler or mesh in the shielding gasket is different from either of the substrates a galvanic potential is established across all three elements, often causing the gasket itself, and eventually the entire assembly, to corrode.
Coatings comprising polymers and nickel filler have been applied to the substrates in an attempt to improve shielding and reduce corrosion. This approach is capable of providing acceptable shielding effectiveness of the assembly, but under conditions in which the coating remains conductive it has little or no effect on corrosion protection.
Some success in preventing corrosion has been obtained by sealing the interface with polythioether or polysulfide sealants containing chromate corrosion inhibitors. However, it is difficult to handle and to dilute these systems enough to use them as sprayable coatings, which are generally preferred for their ease of application. Further, they do not exhibit good thermal stability and become brittle over time.
Thus it is desired to provide a coating which will resist the corrosion of conductive substrates, even in severe environments, while maintaining good EMI/RFI shielding. It is further desired to provide such a coating without the use of hazardous chromates.